Gas turbine engines operate to produce mechanical work or thrust. One type of gas turbine engine is a land-based engine that has a generator coupled thereto which harnesses the mechanical work for the purposes of generating electricity. A gas turbine engine comprises at least a compressor section having a series of rotating compressor blades. Air enters the engine through an inlet and then passes through the compressor, where the rotating blades compress the air and raise its pressure. The compressed air is then directed into one or more combustors where fuel is injected into the compressed air and the mixture is ignited. The hot combustion gases are then directed from the combustion section to a turbine section by a transition duct. Depending on the geometry of the gas turbine engine, often times the combustion section is located radially outward of the inlet to the turbine section, and therefore the transition duct must change in radial profile. However, a change in geometry for the transition duct, which is operating at extremely high temperatures, can create high thermal and mechanical stresses in the transition duct.
By nature, the transition duct has a series of natural operating frequencies and bending modes. The gas turbine engine and combustion system also have a natural frequency, and orders of the natural frequency (i.e. 1E, 2E, 3E, etc). When a component, such as a transition duct, has a natural frequency or mode that coincides with or approaches an engine natural frequency or order thereof, the component can become dynamically excited. If care is not taken to avoid the crossings of these frequencies, operating at these frequencies, or minimizing the time for the crossing, the component may experience excessive wear or failure due to the vibratory stress that occurs when operating at or near the natural frequency of the gas turbine engine or combustion system.